a) Field of the Invention
This invention relates generally to fall protection devices and, more particularly, to energy absorbing devices for use in fall arrest systems. Specifically, the present invention relates to both in-line as well as horizontal life line energy absorbing devices.
B) Description of Known Art
Fall protection and safety equipment are utilized in situations which require an individual to be suspended from an elevated position for a variety of reasons, such as for work at a location below that point. In addition, such equipment is generally used in other situations such as rescue operations, mountain climbing and in numerous other applications where an individual""s entire body must be supported. Such equipment can include harnesses, safety hoists and the like.
One particular type of such fall protection equipment also includes shock absorbers or energy absorbers. Such shock absorbers or energy absorption devices can be utilized in line with the individual""s support cable or in conjunction with horizontal life lines. The purpose of such energy absorption devices is to control the line tension created in horizontal life lines or vertical support lines in personnel fall arrest.
For example, horizontal life lines are particularly useful in certain applications for fall arrest because they do not require a rigid structural support over the work area, but rather can be supported by structure at each end of the work area, such as vertical I-beams at each end of an open flooring or canyon walls on each end of a bridge. The shock absorber performs four main functions within the horizontal lifeline system. First, it adds hysterisis to the system; second, it adds energy capacity to the system; third, it elongates the line to decrease low sag angle load amplifications and, fourth, it can be used to xe2x80x9ctunexe2x80x9d a Horizontal Lifeline to cause the line to absorb energy at a higher rate, thus decreasing both, input energy and total fall distance. Perhaps the most important feature about the design of a Horizontal Lifeline shock absorber is that it must elongate at a high enough force such that it does not allow the falling weight to accelerate and gain input energy any longer than necessary in the fall cycle. In other words, it must provide sufficient initial line tension to reverse the force vector of the falling weight by causing the upward force due to line tension to be equal to the falling weight. If one does not allow unnecessary energy to enter into the system at the beginning of a fall cycle, it will not be necessary to take the energy out at the end. For this reason shock absorbers are designed to work with specific types of cable as compatible components and no substitutions can be made.
There are a number of devices that are designed to be shock or energy absorbers of the type described above. Such devices are disclosed in U.S. Pat. No. 5,598,900, No. 5,423,400, No. 5,433,290, No. 4,396,096 and No. 4,275,802. Such devices are designed to absorb energy while elongated during a fall arrest. A problem with certain of these devices is that they are large, cumbersome, and due to their design, are prone to malfunction by jamming, for instance. Moreover, they do not absorb sufficient energy at the beginning of the fall cycle to significantly reduce final line tension. In addition, some such prior devices do not permit the installer of the fall arrest system to pretension or tune the cable to operate in the correct force versus elongation range. Consequently, the distance a person falls during a fall arrest can be greatly increased. Thus, there remains a need for an energy absorbing device for use with in-line or horizontal life lines which not only absorb significant amounts of energy but also permit high level pre-tensioning of the fall arrest system to reduce the energy allowed to input into the system at the beginning of the fall cycle. The importance of this feature and need is that the quicker in a fall cycle that a falling weight is decelerated, the lower the total energy input and the lower the resultant line tension at the end of the fall arrest cycle.
The importance of the need to start the energy absorption as early as possible in the fall cycle can be understood by reflecting on the fact that the amount of kinetic energy gained by a falling body continuously increases until the body in motion begins to decelerate. Therefore, in order to minimize the amount of energy needed to stop the fall of a person, for example, it is imperative that a significant decelerating force be applied as soon as the fall begins. Accordingly, in systems where the fall of a person is to be decelerated by means of a horizontal lifeline, it is important to take into consideration the elastic and energy retention properties of the cable being used at the horizontal lifeline. Most cables used as horizontal lifelines exhibit highly elastic deformation when resisting the load imposed by a falling person. This elastic deformation is detrimental to the safe deceleration of the falling individual since the elastic deformation simply stores the energy of the fall, and then returns the energy in the form of rebound energy. This rebound can create forces as high as 90% of the initial fall, greatly increasing the chance of injury to the falling worker.
The force needed to stretch an elastic element-such as an elastic horizontal lifeline is proportional to the spring constant of the lifeline times the amount of distance of stretch already imposed on the elastic element. Therefore, to quickly remove fall energy by means of the horizontal lifeline, it is important to pre-load the horizontal lifeline such that additional stretching of the horizontal lifeline will carried out at a much higher energy level than required if the horizontal lifeline had not been pre-loaded. This means that a greater amount of energy is absorbed for a given amount of elastic deformation and loading in the horizontal lifeline. The rapid removal of energy avoids long elastic deformation which in turn reduces the total fall distance and subsequent clearances required.
Still further, the prevention of the rebound action is also assisted by the use of a telescoping energy absorbing shock damper that adds hysterisis to the system. This is a constant force shock absorber that converts all energy into heat while damping the system. A significant problem to be solved by energy absorbing systems that use telescoping components is that these devices use a pair of telescoping components that absorb or use energy by cold working and deforming a section of the components as they move relative to one another. An important limitation with these systems has been that the parts tend to seize against one another while they move relative to one another. Naturally, once the parts seize relative to one another, the safety function of the device is defeated.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention as embodied and broadly described herein, an energy absorption device is disclosed for use both as in-line and horizontal life line shock or energy absorbers for fall arrest systems. The device includes a hollow metal sleeve through which an internal element with an enlarged portion of reduced stiffness. This enlarged portion with reduced stiffness can be formed, for example, from an angularly-shaped solid element, such as a bolt head. The reduction in the stiffness of the bolt head can be accomplished by boring out the head in order to allow the bolt head to flex in a generally radial manner inwardly as it is pulled through the metal sleeve. The angularly-shaped element is multiple-sided and deforms the interior metal surface of the sleeve, thus converting energy of motion and friction into heat and metal deformation work energy as the device is elongated during a fall. The flex of the bolt head prevents the bolt head from permanently welding itself against the interior of the sleeve as the head is slid through the sleeve. In other words, the flexing allows the bolt head to break free the instant any welding begins to take place. This shock absorber is in essence a flexible hex-shaped inner-tube being pulled through a round rigid outer tube. The friction produced by this pulling converts most if not all of the kinetic energy into heat while providing a constant resistance force, so that the shock absorber absorbs the rebound energy of the cable, damping the system to reduce or eliminate rebound.